1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly to a transmitter and receiver using transmitting antenna diversity to combat degradation due to fading.
2. Description of the Related Art
One of the effective technologies for mitigating the effect of multipath fading in a wireless communication system is time and frequency diversity. A technique based on space-time block codes proposed by Vahid Tarokh et al., among techniques associated with antenna diversity, has been extended from a transmitting antenna diversity scheme proposed by S. M. Alamouti so that two or more antennas can be used. Vahid Tarokh et al. proposed and disclosed “Space-Time Block Codes from Orthogonal Designs”, IEEE Transactions on Information Theory, vol. 45, pp. 1456-1467, July 1999, and S. M. Alamouti proposed and disclosed “A Simple Transmit Diversity Technique for Wireless Communications”, IEEE Journal on Select Areas in Communications, vol. 16, pp. 1451-1458, October, 1998.
FIG. 1 is a block diagram illustrating the configuration of a conventional transmitter using the space-time block codes proposed by Vahid Tarokh. As shown in FIG. 1, the conventional transmitter includes an S/P (Serial-to-Parallel) converter 110 and an encoder 120. Here, the transmitter is based on a structure using four transmitting antennas 130, 132, 134 and 136.
Referring to FIG. 1, the S/P converter 110 combines input symbols in units of four symbols and then provides the combined symbols to the encoder 120. The encoder 120 configures eight symbol combinations using the four symbols, and transfers, to the four transmitting antennas 130, 132, 134 and 136, the eight symbol combinations during eight time intervals. The eight symbol combinations can be expressed as an 8×4 code matrix as in the following Equation 1.
                              G          4                =                  (                                                                      s                  1                                                                              s                  2                                                                              s                  3                                                                              s                  4                                                                                                      -                                      s                    2                                                                                                s                  1                                                                              -                                      s                    4                                                                                                s                  3                                                                                                      -                                      s                    3                                                                                                s                  4                                                                              s                  1                                                                              -                                      s                    2                                                                                                                        -                                      s                    4                                                                                                -                                      s                    3                                                                                                s                  2                                                                              s                  1                                                                                                      s                  1                  *                                                                              s                  2                  *                                                                              s                  3                  *                                                                              s                  4                  *                                                                                                      -                                      s                    2                    *                                                                                                s                  1                  *                                                                              -                                      s                    4                    *                                                                                                s                  3                  *                                                                                                      -                                      s                    3                    *                                                                                                s                  4                  *                                                                              s                  1                  *                                                                              -                                      s                    2                    *                                                                                                                        -                                      s                    4                    *                                                                                                -                                      s                    3                    *                                                                                                s                  2                  *                                                                              s                  1                  *                                                              )                                    Equation        ⁢                                  ⁢        1            
In the above Equation 1, G4 denotes the code matrix associated with the symbols to be transmitted from the four transmitting antennas, and s1, s2, s3 and s4 denote four input symbols to be transmitted.
As described above, the encoder 120 performs negate and conjugate operations for the four input symbols, and then outputs, to the four antennas 130, 132, 134, and 136, symbols generated after the negate and conjugate operations during the eight time intervals. Here, symbol sequences of the matrix's rows outputted to the respective antennas are orthogonal to each other.
Explaining in detail, the four symbols s1, s2, s3 and s4 of the first row are transmitted to the four antennas 130, 132, 134 and 136 during the first time interval. Similarly, four symbols −s*4, −s*3, s*2 and s*1 of the last row are transmitted to the four antennas 130, 132, 134 and 136 during the last time interval. In other words, the encoder 120 sequentially transfers symbols of the mth column of the code matrix to the mth antenna.
FIG. 2 is a block diagram illustrating the configuration of a receiver receiving signals from the transmitter shown in FIG. 1. As shown in FIG. 2, the receiver includes a plurality of receiving antennas 140, 145, etc., a channel estimator 150, a multi-channel symbol arranger 160 and a detector 170.
Referring to FIG. 2, the channel estimator 150 estimates channel coefficients indicating gains of channels from the transmitting antennas 130 to 136 to the receiving antennas 140, 145, etc. The multi-channel symbol arranger 160 collects symbols received by the receiving antennas 140, 145, etc. and then provides the collected symbols to the detector 170. The detector 170 multiplies the received symbols by the channel coefficients, thereby producing hypothesis symbols. The detector 170 calculates decision statistic values for all possible symbols using the hypothesis symbols and detects desired symbols on the basis of threshold detection.
Alamouti's space-time block code technique can obtain the maximum diversity order corresponding to the number of transmitting antennas without lowering a transmission rate although complex symbols are transmitted through two transmitting antennas. The transmitter and receiver proposed by Tarokh, shown in FIGS. 1 and 2, can achieve the maximum diversity order using space-time block codes based on a matrix having orthogonal rows, through the extension of Alamouti's space-time block code technique. However, only half the transmission rate can be achieved since the transmitter transmits the four complex symbols during the eight time intervals. Because the eight time intervals are needed to completely transmit four-symbol blocks generated by encoding the four input symbols, reception performance due to the change of a channel environment for block symbols is degraded in the case of fast fading.
There are problems in that the transmission rate is degraded and hence the latency is lengthened since 2N time intervals are needed where N complex symbols are transmitted through four or more antennas.